The present invention is directed to a non-anodized sheet product having at least one treated surface for use in a lithographic plate.
The present invention is directed to improving the surface of metals and their alloys, particularly aluminum and aluminum alloys, so that a variety of materials may be bonded, attached, adhered, mated, held, either permanently or temporarily, to the surface of the finished product which may be a foil, a sheet, a plate, a composite or laminate. Such finished products include flexographic plates and lithographic plates. These products typically require use of a rolled aluminum sheet.
Rolled metal alloy sheet and plate have an identifiable bias. The rolling activity creates roll grind, which appears as striations that ran lengthwise in the rolled product. These striations may interfere with certain downstream activities such as lithography. In lithography, for example, the sheet surface is coated with a photosensitive or laser-imagable coating. The run life and printed image definition are controlled by the bond strength of the aluminum to the coating and the directionality of the resultant product. The presence of roll grind in these products results in an end product with low run life and poor definition due to poor bond strength and directionality. To combat these effects typical lithographic plate manufacturers chemically or mechanically grain the surface of the aluminum to create more extended surface area to enhance the bond strength of the lithographic plate to a printing composition and reduce the directionality in the product.
Conventional treatment of aluminum substrates for adhesion of a printing composition, such as photosensitive coating, thereto includes graining and anodizing the surface of the substrate onto which the photosensitive coating is to be applied. Anodization typically produces alumina on the surface of the substrate. In addition, the aluminum surface may be post-treated to improve the adhesion of a photosensitive coating to the aluminum substrate while also creating an oleophobic surface. Graining and anodizing and/or post-treating (e.g. silicating) an aluminum substrate require specialized processing steps. Graining may be performed electrochemically or mechanically (e.g. brush graining). These processes add significantly to the time and expense of preparing an aluminum substrate for coating with a photosensitive material.
Accordingly a need remains for a sheet which minimizes or avoids graining and anodizing and/or post-treating processes and which readily retains coatings thereon for the manufacture of a lithographic sheet or plate.
This need is met by the lithographic sheet product of the present invention. The sheet product includes a substrate, preferably made from an aluminum alloy, having at least one surface that is covered with a polymeric pretreatment layer. The pretreatment layer is particularly suited for enhancing the adhesion of a printing composition to the substrate. The printing composition may be a photosensitive coating or ink or the like. Suitable polymers for the pretreatment layer include polymers of acrylic acid or methacrylic acid, organophosphorous polymers and copolymers thereof. The surface of the substrate may be mill-finished or more preferably, is roll textured.
The pretreatment layer may further include about 4 to about 50 wt. % dopant particles of alumina, silica, titanium dioxide, black dye or black pigment or the substrate may be etched or both. When the lithographic sheet product is covered with a photosensitive coating, the dopant particles and the etched surface increase surface area and minimize the gloss and reflectance of light from the pretreatment layer during exposure of the photosensitive coating to imaging light.